This application claims the benefit of U.S. Provisional Application Ser. No. 60/303,023 filed Jul. 5, 2001 and Japanese Application 2001-053,188, filed Feb. 27, 2001, the entireties of which are incorporated herein by reference.
(1) Field of the Invention
This invention relates to a method for fabricating a nitride film, particularly a III nitride film usable as an underfilm of a semiconductor element such as a light-emitting diode or a high velocity IC chip.
(2) Related Art Statement
III nitride films are employed as semiconductor films for photonic devices constituting a light-emitting diode, etc., and recently, win a lot of attention as semiconductor films for electronic devices, for example, constituting high velocity IC chips to be used in cellular phones.
Such III nitride films are usually fabricated by MOCVD methods. Concretely, a substrate on which III nitride films are formed is set onto a susceptor installed in a given reactor, and then, heated to 1000xc2x0 C. or more with a heater provided in or out of the susceptor. Thereafter, raw material gases are introduced with a carrier gas into the reactor and supplied onto the substrate.
On the substrate, the raw material gases are dissolved through thermochemical reaction into constituent elements, which are reacted to deposit and fabricate a desired III nitride film on the substrate.
The lattice constant of the III nitride film is affected largely by the composition of the film itself. Therefore, if the composition of the III nitride film is selected, the difference in the lattice constant between the substrate and the III nitride film may become large, so that many misfit dislocations may be created at the boundary between the substrate and the III nitride film.
In this case, if another III nitride film is epitaxially grown on the III nitride film, a large amount of dislocation density of about 1010/cm2 may be created in the epitaxially grown film because the misfit dislocations are propagated. As a result, the crystal quality of the epitaxially grown film may be deteriorated, and the electrical properties and the optical properties may be deteriorated.
In order to solve the above problem, such an attempt is made as fabricating a patterned mask made of SiO2 on a substrate and epitaxially growing a given III nitride film laterally on the mask. In this case, misfit dislocations originated from the interface between the substrate and the film are not propagated vertically but propagate laterally above the mask. As a result, the threading dislocation density above the mask is reduced.
In the fabrication of the patterned mask, however, since a photo-lithography process including an etching operation is required, the total fabrication process for the III nitride film may become complicated.
It is an object of the present invention to provide a method capable of easily fabricating a III nitride film with a lower dislocation density.
In order to achieve the above object, this invention relates to a method for fabricating a III nitride film and includes the steps of forming a lower region having a composition of Alx1Gax2Inx3N (x1+x2+x3=1, 0.5xe2x89xa6x1xe2x89xa61.0), and forming an upper region having a composition of Aly1Gay2Iny3N (y1+y2+y3=1, 0xe2x89xa6y1xe2x89xa6x1xe2x88x920.1). A boundary face is created and divides a given III nitride film into the lower region and the upper region, with the Al content difference being 10 atomic percent or more between the lower and upper regions.
The inventors intensely studied to obtain a III nitride film with lower dislocation density through an easy fabricating process without the need to use a patterned mask made of SiO2. As a result, they found out that by forming the above-mentioned boundary face in the III nitride film, the dislocation density of the III nitride film can be reduced, and thus, the crystal quality of the III nitride film can be improved.
FIG. 1 is an explanatory view of a fabricating method of a III nitride film according to the present invention. In a III nitride film 1 including Al element shown in FIG. 1, a boundary face 2 is formed and thus, the III nitride film is divided into two regions 3 and 4. The Al content of the upper region 4 is lower than the Al content of the lower region 3 by 10 atomic percent or more. Herein, the Al content of the lower region 3 is set to 50 atomic percent or more. In this case, the Al content as mentioned above is defined as the amount included in all of the III elements in the III nitride film.
The III nitride film 1 is fabricated on a given substrate (not shown), and thus, many dislocations may be created, which originated from the dislocations contained in the substrate and the misfit dislocations between the substrate and the III nitride film 1 due to the large difference in lattice constants.
However, the dislocation propagation in the III nitride film can be inhibited by the boundary face 2. Therefore, if many dislocations are propagated in the III nitride film 1, they can not be propagated beyond the boundary face 2, and thus, remain only in the lower region 3 or at the boundary face 2. As a result, the dislocation density of the III nitride film can be reduced in the upper region 4.
As a result, the crystal quality of the upper region 4 of the III nitride film 1 can be improved. Therefore, if a given function is provided in the upper region 4 in advance, the III nitride film 1 can be employed as a given semiconductor film entirely.
Particularly, the III nitride film 1 is preferably used as an underfilm of a semiconductor element such as a semiconductor light-emitting element, e.g., a light-emitting diode or a high velocity IC chip. In this case, since another semiconductor layer is fabricated on the upper region 4 of the III nitride film 1, which has a good crystal quality, the crystal quality of the semiconductor layer can be improved. As a result, the luminous efficiency or the high velocity response of the semiconductor element can be remarkably enhanced.